This machine is about 200 flights old. One fine day, in mid flight, it started pitch pumping on its own - was totally out of control.
It seems the servo was malfunctioning


I have done this test earlier too, but this kind of twitching never showed up during bench tests. Now, it is has started showing up.

That is some serious servo twitching. That one servo (elevator) has started twitching after a good spool up which means underload or probably due to static discharge from the tail belt (in case of belt drive), motor/electronics. Check for any damage to the servo wire, damage to the motor/ESC wires and try to cure the static discharge of the tail belt. Before doing anything I would try a different servo and see... could be a bad servo with bad centering underload.

Static discharge was what I was looking for too. I changed to Align belt, lubed it with silicone oil etc. I suspected the ESC too. Thought it might be malfunctioning and interfering. What I have in the video is brand new ESC.
But the servos are about 6 months old with 200 flights on them. I have seen a total of 3 TGY 306HV servos going bad now. They work awesome initially, but they just don't last. Either they go dead one fine day or they start malfunctioning, like in this case.
Its time for MKS DS92A+

I bought 3Tgy From Hk and one them twitches at some intervals (no flights ) . Thought it might be the fbl system or the esc but it twitched with others too.. I'm on torque tube ..

I am suspecting a bad servo.

One other thought: which radio system are you using? Most 2.4Ghz transmitters very close to the receiver will make servos glitch. Keep your radio a few feets away from your model.

Sujit i still suspect interference from some place ,does the twitch happen only when the heli is spooled or does it happen when the rotor is not spooled ,also try connecting all wires only to reciever and then spool up and check if its happening again new trick ..........

might be a Rx problem.

Shannon, the twitch is unpredictable. It can happen anytime - whether head is spining or not. I had to try a few times to get it in video.
But, If I push the elevator servo with my finger, it will give a twitch in opposite direction (of force). I took out this servo and verified it on a servo tester. This problem is replicated 100%. As soon as I push it, it will twitch. It may or may not continue the twitching.

I had a spare servo, so I could replace it on my heli. I did 4 flights today, there was no problem. I am 99% sure, problem was with the servo. There was no interference or anything.

I suspected interference because when the twitch happens in flight, the heli does not move in one direction - it actually sort of pitch pumps. So, I my assumption was all three servos are acting wierd, so it has be some sort of interference, which was affecting all servos.
But, someone at helifreak told me that even if one servo was malfunctioning and pulling the heli in one direction, the FBL unit, will detect the movement and correct it. So the end result will look pitch pumps - what do you think about this? - i am not 100% sure on this behaviour

Hmm, I havnt seen an servo glitch because of proximity to tx
I am using a single satellite on the heli. JR EA101, 2.4Ghz DSM2

I have seen this happening a few times in Spektrum system... unable to bind, takes too long bind or servos glitch.

From the Spektrum DX6i manual:

Tips on Using 2.4GHz Systems:

Question: Sometimes the system takes longer to connect and sometimes it doesn’t connect at all. Why?

Answer: In order for the system to connect (after the receiver is bound) the receiver must receive a large number of continuous (one after the other) uninterrupted perfect packets from the transmitter in order to connect.
This process is purposely critical of the environment ensuring that it’s safe to fly when the system does connect. If the transmitter is too close to the receiver (less that 4 feet) or if the transmitter is located near metal objects (metal transmitter case, the bed of a truck, the top of a metal work bench, etc.) connection will take longer and in some cases connection will not occur as the system is receiving reflected 2.4GHz energy from itself and is interpreting this as unfriendly noise.

ok, didn't know about it - thanks for the info!

Here's a couple of things you can try

for interfence - if your servo leads are long enough
try putting a ferrite choke on the lead.

if the servo still glitches - most likely the feedback pots
are dirty - open up the servo and clean the pot with methylated
sprits... this cures 99% of the glitching

Saikat, can you please advise how we clean the feedback pots or links to any videos.
Thank you.
I have another servo gone bad (after 360 flights). I can see a small twitch at the center position.

Raja the  - the feedback pot is  attached to to output arm of the
servo.  Open the servo and spray CRC-26(it is a electrical speciality spray) liberally
you can spray it over the electronics too. 99% it will work - CRC 26 will be available
in any shop that repairs electronics - you do not have to buy the whole can

let us know if it works out.

Thanks for the tip saikat. I finally found CRC2-26 (brand is different though XXX2-26) for 220.00 Rs (for a can)
But, the twitching reason was infact the board shorting out because of the aluminum case. Once the internal circuit board was covered with electric tape, the issue never re-occured. This is a well known issue, but I ignored it because it never created problems initially.
Anyway, the design of this particular type of servo is such that, insulating the board is mandatory - learnt it the hard way.

EDIT Infact on opening other two servos, I found one of them had a insulation tape from the factory. So, it definately looks like a QA issue - some servos have insulation and others dont

Now Toh I should definitely insulate them .. Shannon warned me once.!!... Thanks for confirmation Sujith sir

am gonna say it here haha sujit i told u so lol

yeah ..... 

dhruv u brat u doubted my intellect

Let me correct that shannon, its expert intellct or heli pro intellect

intellect in general 

BTW the spray mentioned by saikat - CRC2-26 is really awesome!. The ones available in India go a slight different name - DC2-26 something, dont remember exactly
The button on my accucel-8 charger had started giving problems. I sprayed CRC2-26 on the buttons and kept it for a while. After that, the buttons have become 95% OK.
Similarly, i feel this spray has more penetrative capabilities compared to WD40 especially when working with small tools.
Great item - highly recommeneded

Point Noted Down
And you need to open the case of charger then spray it?

No need to open the charger - just spray in the gap (between the buttons)

Oh okay got it
Thanks